We start with our chief investigative correspondent, brian ross. Brian? Reporter: Good evening. The headlines say the nightmare in boston is over. That's hardly the case for the investigation as the fbi now seeks the motive of the accused suspect and his dead brother. Whether they were in league with others here or overseas. As police and federal agents closed in on the accused bomber last night, with a helicopter beaming back these remarkable thermal images of him hiding in a boat. We have movement in the boat. He just sat up. He is moving, and flailing about, quite a bit of movement. tsarnaev alive. Even after exchange of gunfire. All units, do not return fire. Reporter: From the white house down, the priority now is to learn the motives of the two brothers who authorities say are responsible for the successful terror attack on the boston marathon. We will determine what happened. We will investigate any associations that these terrorists may have had. Reporter: Tsarnaev was taken to the hospital with serious injuries. But once he's conscious, they plan to question him on the -- under what is known as the public safety exception, not telling him he has a right to remain silent, or to have a lawyer. In previous cases for a duration of about 50 minutes. There's a public safety exemption in cases of national security and acts of terrorism. And so the government has that opportunity. Reporter: The dramatic events unfolded last night after authorities had lifted a lockdown on watertown, when the man who lives in this house, dave henneberry, was finally able to go outside, he spotted blood on his boat in the back driveway. According to his neighbor. He started to open up the zipper, looked inside and he saw blood. Then he looked and his deck has a box, the engine box is up above the deck. And he looked at what he thought was a body right behind the box. Reporter: Fbi and police s.W.A.T. Teams stormed the area and began a two-hour stand youf, with gunfire exchanged at one point. The helicopter beamed back those images showing the suspect in the boat. We have movement. Reporter: A robot was sent in to rip the tarp off the top of the boat. If they're able to successfully rip that off, he'll be fully exposed. Reporter: Tsarnaev had been injured in a gun fight with police the night before and appeared weak and not wearing a suicide vest as had been feared. The hostage rescue team did work in trying to negotiate him out of that boat. Reporter: And then it was over. The suspect's in custody. Reporter: Across the street, across boston, and across the nation, the cheers went up. Tonight tsarnaev is here under guard, according to the governor, in serious, stable condition, but still not able to communicate. So when he's able to communicate, police are going to have a limited period of time, maybe 50 minutes, to interview the suspect before they have to read him his rights. What are the key questions they're likely to ask? There's two key questions. Are there others working with him and are there other targets that still might be hit by others working with him? Dan? Brian ross, we thank you once again. Now to the suspects, there are new details tonight about

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